'Twin Corresponding Sudokus II' printed from http://nrich.maths.org/

Twin A

Twin B

You can download a word
document containing both the sudoku gridshere.

Rules of Twin Corresponding Sudokus

The object of this sudoku variant is to fill in the whole of each
9x9 grid with numbers 1 through 9 (one number per cell) so that
each row, each column, and each of the nine 3x3 squares contain all
the nine different numbers 1 through 9.

This Sudoku problem consists of a pair of linked standard sudoku
puzzles each with some starting digits.

To get a complete solution for the twin puzzles, we have to solve
each twin puzzle with the usual strategies but we can only get the
complete solution for the twin puzzles by substituting the
corresponding values of the digits from one twin sudoku into the
other.

For example, we can see that in r4c9, that is row 4 column 9, twin
A contains the number 3 and twin B contains the number 8. This
means that everywhere the number 3 appears in twin A, the number 8
should appear in the corresponding position in twin B and vice
versa. This immediately gives you some more starting digits in each
twin! It is not necessary for a number in one twin puzzle to be
different from its corresponding counterpart in the other twin
puzzle.